Field of the Invention
The present invention relates to flow separation detectors and, more particularly relates to feedback sensor arrangements adapted to provide for the measurement of surface aerodynamic flow phenomena, and especially with regard to aerodynamic flow separation which is encountered over a surface. Moreover, the invention is also directed to aspects which facilitate the detection of aerodynamic flow separation with a concurrent detection of encountered mechanical strain and stresses in the surface structure being monitored.
Currently, various types of detection or sensor systems are being investigated for their applicability to the technology concerning problems which are being encountered as a consequence of aerodynamic flow separation; for instance, such as during airflow over the wing surfaces of an aircraft, and which may have a important bearing on and potentially adversely influence the performance of the aircraft. For example, some of the systems being investigated provide for a so-called closed-loop control of aerodynamic flow separation, which necessitate the provision of feedback sensors which are sensitive to flow separation, and whereby such sensors are typically required to be surface-mounted on the surface or wall which is subject to aerodynamic flow separation. At this time essentially fully developed and commercially available sensors employed for this purpose are pressure transducers which are capable of measuring surface aerodynamic phenomena and flow separation parameters.
In particular, types of sensors which are adapted for the investigation or measurement of aerodynamic flow separation which takes place on a surface or wall may be so-called electronic xe2x80x9cthermal tuftxe2x80x9d sensors. Thus, in essence, thermal tuft sensors may be generally constituted of one or more electrical heating elements with temperature sensors being mounted spaced upstream and downstream thereof along the presumed directions of aerodynamic flows passing over a surface. Generally, the flow separation, encountered in at least a two-dimensional flow, is defined by a location wherein the flow proximate a wall over a surface tends to oppose a primary flow direction; pursuant to a phenomenon referred to as a backflow. Thus, the thermal tuft sensors are spacedly mounted in the presumed flow direction. The electrical heating elements are normally pulsed on and off, thereby heating a local packet of fluid providing the aerodynamic flow. Depending upon the local instantaneous direction of the flow, either the upstream or downstream located temperature sensor will detect a rise in temperature as the heated packet of fluid is convected there past. Generally, the pulses are counted as a measure of the percentage of the time during which the flow is either upstream or downstream in its direction. Alternatively, the time internal between the heater element actuation and sensor detection can be recorded as a measure of near-wall upstream or downstream velocity magnitude.
Such electronic xe2x80x9cthermal tuftxe2x80x9d sensors are extensively described, in an article by Shivaprasad and Simpson entitled xe2x80x9cEvaluation of a Wall-Flow Direction Probe for Measurements in Separated Flowsxe2x80x9d, published in the Journal of Fluid Engineering, 1981. In that instance, a pair of thermal sensors are spaced along a surface whereby a free stream of a fluidic or airflow may have a flow direction extending across the locations of the sensors. A plurality of heaters are interposed between the sensors, and further heaters are arranged offset aside the directional flow so as to be able to determine aerodynamic separation or, in essence, a breakdown of a boundary-layer flow of fluid passing across the surface which may pass either directly across the sensors or at an angle relative thereto. These sensors are electronically connected to the electrical or electronic circuitry of a device which; for example, may be a part of an aircraft electrical operating system.
Although the foregoing thermal tuft sensors are generally satisfactory in operation in detecting flow separation phenomena, they require the input of electrical energy from the electrical components of various devices, or in connection with aircraft from the electrical aircraft system network, thereby representing a source for electrical energy drain and consumption.
More recently, in order to obviate or ameliorate the electrical energy requirements in the provision of feedback sensor arrangements, particularly such which are employed for a closed-loop control of aerodynamic flow separation; for instance, that on the wing of an aircraft wherein there can be encountered a breakdown of a boundary-layer flow which may adversely affect the performance of the aircraft, there has been developed a system of flow separation sensors which are based on fiber optics and which may be employed for separation feedback control. In that connection, reference may be had to the copending Wetzel, et al. U.S. patent application Ser. No. 09/396,472, now U.S. Pat. No. 6,380,535, entitled xe2x80x9cOptical Tuft for Flow Separation Detectionxe2x80x9d; commonly assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. In particular, the sensors which are based on fiber optics may employ an optical tuft arrangement based on the thermal/fluidic principles of the electrical thermal tuft, but with the employing of fiber optics signal and energy transmission instead of electronics. To that effect, the light transmitted through the fiber optics is adapted to be converted into heat enabling a packed of heated fluid to be convected in the direction of a predominant aerodynamic flow, and to impact or contact one of the temperature sensors which are based on fiber optics at a small following time interval, so as to provide the required information concerning aerodynamic flow separation.
Although various other types of sensors have been developed which are based on fiber optics, these are primarily employed for the measurement of strain, acceleration and temperature, and currently there is also known the development of new pressure transducers in the technology. However, none of these sensors in themselves are designed for flow separation detection, particularly for use in the closed-loop control of aerodynamic flow separation, or for investigations of breakdown phenomena in boundary layer flow situations.
Accordingly, in order to substantially improve upon the current state of the technology, the present invention utilizes the end of an optical fiber as a tuft in itself, and optically transduces the movement of the fiber into a useable signal for flow separation detection.
The invention contemplates for a multitude of tufts to be placed on the surface (e.g., the surface of awing), with the tufts made out of an appropriate optical fiber material. The length of the optical fiber exposed to the airstream should be short, on the order of 0.1 to 1 inches, and the optical fiber should be very fine so as to be flexible, with diameters on the order of 0.001 to 0.01 inches. One embodiment calls for the fibers to be flexible enough to bend 90xc2x0 or more (thus longer, with a smaller diameter), in which case the large flow fluctuations of a separated flow will result in dramatic tuft motion. Another embodiment calls for the tuft to be relatively stiff (thus shorter, with a larger diameter), in which case the large flow fluctuations of a separation flow will result in large optical fiber stresses.
The length of fiber that is not exposed to the aerodynamic flow can be packaged numerous ways. One embodiment calls for the fibers to be embedded in the skin material of the surface (especially when the skin is made of a cured composite material). Alternatively, the fibers can run inside the structure of the device (e.g., along the internal structure of a wing), and exit the wing surface through holes. Also, the fibers could be embedded in a low-profile tape, which can be applied to the surface, and can thus be easily replaced if damaged in service.
The optical fibers from many tufts are thus run to a central processing location, and either interface to a multitude of processing stations, or interface with an optical multiplexer which is interfaced to one processing station. Therefore, this sensor requires no electronics local to the sensor, and is in essence purely optical.
The strain and curvature induced by the air motion results in birefringence in the optical fiber. In a fiber loop or a fiber with a reflective end, the birefringence can be detected as a change in either the phase or the intensity of the transmitted light. Even a partially reflective end would be sufficient to detect the excess fiber losses induced by the curvature effects. Using fiber-optics to measure strain induced in the fiber is commonly practiced, and is described in many references (e.g., Optical Fiber Sensors, ed. By J. Dakin and B. Culshaw, 1997). In low speed flow, or in flow streams with weak transverse separations, the cross-sectional area of the fiber could be increased in the direction of interest to increase the drag and thus the sensitivity of the fiber tuft Likewise, other techniques can be used to increase the fibers"" strain sensitivity, including fiber grating and tooth blocks.
When a tuft is in attached flow, in effect no flow separation, its motion is relatively small and predominantly in the flow direction. Likewise, the signal generated will be relatively constant and should be close to the rest value (defined for purpose of argument to be xe2x80x9czeroxe2x80x9d). The tuft in a separated zone will oscillate wildly, and typically have some mean direction that is not in the predominant flow direction. Likewise, the signal generated by this tuft will oscillate wildly and have some mean offset different than zero. Either the offset mean or the degree of variation of the signal can be used as indications of the presence and integrity of flow separation.
Alternatively, the ends of the fiber can be made to be transmissive, and one could view the light emitted from the fiber ends from a camera external to the test surface. If, for example, the camera were placed above a wing that has attached flow, the fibers would point downstream and their emitted light would not be visible to the camera. When the flow on the wing separates, the ends of the tufts will sometimes point upwards towards the camera, resulting in an image that indicates regions of separation. This could be very effective in wind tunnel testing, but may also be adaptable to aircraft or to other devices with flow separation.
In addition to the foregoing, the present invention has also further potential applications with regard to the optical fiber sensor aspects.
Additionally, the invention expands on the potential application for the sensors. In the present disclosure there is described using the sensors primarily as feedback for active flow control devices. However, even on an aircraft with no active flow control, these sensors could be of tremendous value. High-maneuverability aircraft, especially fighter aircraft, attempt to compensate for the stability or instability of the aircraft at any instant in time electronically. The closer one can keep an aircraft neutrally stable without becoming unstable, the more maneuverable will be the aircraft. Classical control systems in such aircraft are based on signals that one can reliably measure: accelerations, velocities, etc. Then, one attempts to relate these signals to the instantaneous aerodynamic forces and moments acting on the aircraft, and make control decisions based on this assumed aerodynamic state. In actuality, the true aerodynamic state in very severe maneuvers is very hard to adapt so as to predict from these inertial measurements. However, if one could measure the true instantaneous flow separation location and extent, with a sensor system such as ours, it is conceivable that such information would allow vastly superior control schemes, and thus superior aircraft maneuverability.
In particular, although generally referred to in connection with feedback sensors for active flow control on the aircraft it is also possible to utilize flow sensing any fluidic device, including gas turbines and aircraft engines, as long as the optical fiber and other exposed components can be designed to withstand the necessary flow temperatures encountered during operation of those particular devices.
A particular aspect of the invention resides in the utilization of the fiber optic tufts for strain sensing of the particular structures on which the optic fibers are located. Although it has already been proposed in the technology to utilize optical sensors on the wings of aircraft to measure strain of the wing structure, pursuant to the invention it is possible to utilize a single or unitary fiber structure to measure both mechanical strain in the structure, such as an aircraft wing, and concurrently flow separation utilizing the inventive techniques. This, in essence, imparts a multiple applicability and faculty to the present invention, heretofore unknown in the prior art.
Accordingly, it is an object of the present invention to provide an arrangement for the investigation of aerodynamic flow separation.
A more specific object of the present invention is to provide a sensor arrangement for the investigation of aerodynamic flow separation utilizing flow separation tuft sensors which are based on fiber optics.
Yet another object of the present invention is to provide for novel feedback tuft sensors which are based on fiber optics, wherein these are employed for aerodynamic separation feedback control, particularly with regard to aerodynamic flow separation taking place on the wing surfaces of an aircraft, and which may also concurrently measure strain and/or stresses acting on the surfaces.
A still further and more specific object of the present invention rises in the provision of feedback sensor arrangements for either the open-loop or closed-loop control of aerodynamic flow separation, which are adapted to extend through or to be positioned within the skin structure of an aircraft wing or airfoil.